Disk drives are computer data storage devices that store data in a plurality of concentric tracks on the surface of a disk. A transducer, such as an inductive head, is used to both write data to said tracks and to read data from said tracks as the disk is rotated beneath the transducer. The transducer is moved over the surface of the disk using an actuator arm to support the head and some form of motor means, such as a voice coil motor (VCM), to impart motion to the actuator arm. A servo control system is implemented to control the motion created by the motor means.
Servo control systems generally perform two distinct functions: seek control and track following. The seek control function comprises controllably moving the transducer from a initial track position to a target track position. In this regard, the servo control system receives a command from a host computer that data is to be written to or read from a target track of the disk and the servo system proceeds to move the transducer to the target track from wherever it is currently located. Once the transducer is moved sufficiently near the target track, the track following function is activated to center and maintain the transducer on the target track until the desired data transfers are completed.
In making the transition between the seek function and the track following function, a settling period occurs during which the transducer converges on the center of the target track. It is not until after the settling period has ended that data transfer between the transducer and the target track can occur. As should be expected, the length of the settling period depends to a large extent on how accurately the seek routine has moved the transducer. In other words, if the transducer is very close to the center of the target track and has a very low velocity at the end of the seek cycle (i.e., the residual velocity and position of the transducer are low), the settling period will be relatively short.
Seek time is a measure of how fast, on average, a disk drive takes to move and settle a transducer over a target track after a command is received from a host computer to access the track. Because speed is a very important attribute in computer systems, seek time is generally used as an indicator of the quality or usefulness of a disk drive. Therefore, it is highly desirable to reduce the seek time of a disk drive as much as possible.